Tau pathology and neurodegeneration

Review

Tau pathology and neurodegeneration Maria Grazia Spillantini, Michel Goedert

The pathway leading from soluble and monomeric to hyperphosphorylated, insoluble and filamentous tau protein is at the centre of many human neurodegenerative diseases, collectively referred to as tauopathies. Dominantly inherited mutations in MAPT, the gene that encodes tau, cause forms of frontotemporal dementia and parkinsonism, proving that dysfunction of tau is sufficient to cause neurodegeneration and dementia. However, most cases of tauopathy are not inherited in a dominant manner. The first tau aggregates form in a few nerve cells in discrete brain areas. These become self propagating and spread to distant brain regions in a prion-like manner. The prevention of tau aggregation and propagation is the focus of attempts to develop mechanism-based treatments for tauopathies.

Lancet Neurol 2013; 12: 609–22

Introduction

Correspondence to: Dr Michel Goedert, MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge CB2 0QH, UK [email protected]

The progressive dysfunction and death of nerve cells characterise the most common neurodegenerative diseases in human beings, including Alzheimer’s disease and Parkinson’s disease. The clinical picture is defined by the affected brain regions, explaining why Alzheimer’s disease is a dementing disease and Parkinson’s disease mainly a movement disorder. No mechanism-based treatments for these diseases are available, and those treatments that exist are symptomatic. Specific protein inclusions define most neurodegenerative diseases at the pathological level. In 1907, Alois Alzheimer in Munich, Germany, and Oskar Fischer in Prague, Czech Republic, described the association between dementia and the presence of abundant neuritic plaques and neurofibrillary tangles visualised with silver stain in the cerebral cortex.1–4 3 years later, Emil Kraepelin, head of the Munich Institute, named a form of presenile dementia after Alzheimer and suggested that Alzheimer’s disease was distinct from senile dementia, which has an onset of disease after 65 years of age.5 In 2013, the Phe176Leu mutation in PSEN1, the gene encoding presenilin 1, was identified in Alzheimer’s first patient Auguste Deter, who had an age of disease onset of 51 years.6 In the 1970s, the general belief was that most patients with senile dementia had similar pathological changes in their brains to patients in their presenium with Alzheimer’s disease. As a result, the idea of Alzheimer’s disease was widened to include cases of senile dementia.7 In 1911, Alzheimer described argyrophilic inclusions in a form of frontotemporal dementia8 that was subsequently named Pick’s disease, after Arnold Pick, head of the Department of Neuropsychiatry at the German University in Prague where Fischer worked. These inclusions are now known as Pick bodies. In 1912, Fritz Heinrich Lewy described the inclusions characteristic of Parkinson’s disease (Lewy bodies and Lewy neurites) in Alzheimer’s laboratory in Munich.9,10 Electron microscopy showed that all these inclusions are made of abnormal filaments. In the past 30 years, a causal connection has been established between the formation of inclusions and the process of degeneration; the molecular components of the inclusions were identified and the causes of rare inherited forms of Alzheimer’s disease, Parkinson’s disease, and www.thelancet.com/neurology Vol 12 June 2013

frontotemporal dementia were discovered. In most cases, pathogenic mutations caused disease through an overproduction of the amyloidogenic protein or an increase in the protein’s propensity to aggregate. Similar toxic mechanisms might underlie the sporadic forms of disease. Neurofibrillary tangles and Pick bodies made of the microtubule-associated protein tau in a hyperphosphorylated and filamentous state form inside some brain cells (figure 1).11 In the 1980s and early 1990s, immunohistochemistry, electron microscopy, biochemistry, and molecular biology were used to establish that the paired helical and straight filaments seen in the brains of patients with Alzheimer’s disease are made of all brain isoforms of the microtubule-associated protein tau in a hyperphosphorylated state.13–20 In the late 1990s, mutations in MAPT, the gene encoding tau, were shown to cause an inherited form of frontotemporal dementia and parkinsonism, with high disease penetrance and characterised by abundant hyperphosphorylated filamentous tau inclusions, proving that dysfunction of tau protein is sufficient to cause neurodegeneration and dementia.21–23 More recently, chronic traumatic encephalopathy, a neurodegenerative disease resulting from environmental causes including repetitive blast or concussive injuries, or both, was also found to be characterised by filamentous tau inclusions (figure 1).12,24,25 The disease was originally described as the clinical deterioration that occurs after repetitive brain trauma in boxers.26 Alzheimer’s disease is defined by the presence of two types of abnormal protein deposits: extraneuronal amyloid β and intraneuronal tau.13–20,27,28 Intraneuronal tau inclusions are also characteristic of diseases associated with extracellular deposits made of integral membrane protein 2 (the Bri peptide)29 and some cases of Gerstmann-Sträussler-Scheinker disease with extracellular deposits of the prion protein.30 The conversion of soluble to insoluble filamentous tau protein is central to many human neurodegenerative diseases (panel); therefore, tau is an excellent potential therapeutic target. We review evidence showing that tau assembly is essential for the pathogenesis of many human neurodegenerative diseases, collectively referred to as tauopathies, and discuss implications for the development of mechanism-based treatments.

John van Geest Centre for Brain Repair, Department of Clinical Neurosciences, University of Cambridge, Cambridge, UK (Prof M G Spillantini FRS); and MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge, UK (M Goedert FRS)

609

Review

A

B

C

20 µm

Figure 1: Abnormal tau deposits (A) Pick bodies and abnormal neuritic inclusions made of tau (brown), as found in some cases of frontotemporal dementia caused by MAPT mutations and in sporadic Pick’s disease. (B) Neurofibrillary tangles and neuropil threads made of tau (brown) in chronic traumatic encephalopathy. (C) Neuritic plaques made of amyloid β (blue), and neurofibrillary tangles and neuropil threads made of tau (brown) in Alzheimer’s disease. Immunohistochemistry with anti-tau and anti-amyloid β antibodies. Scale bar in C applies to panels A–C. Panels A and C reproduced from Goedert and Spillantini,11 by permission of the American Association for the Advancement of Science. Panel B reproduced from Omalu and colleagues,12 by permission of the American Association of Neurological Surgeons.

Tau isoforms and their interactions with microtubules Tau is a natively unfolded microtubule-associated protein that binds to and might have a role in the assembly and stabilisation of microtubules.11 In nerve cells, tau is concentrated in axons,31 but recent work suggests a physiological role for tau in dendrites.32 Six tau isoforms 610

are expressed in the adult human brain, and these are produced by alternative mRNA splicing of the MAPT gene on chromosome 17q21.31 (figure 2).34–36 The tau isoforms differ from each other by the presence or absence of a 29-aminoacid or 58-aminoacid insert in the amino-terminal half and by the inclusion or not of a 31-aminoacid repeat encoded by exon 10 of MAPT in the carboxy-terminal half of the protein. The inclusion of exon 10 results in the production of three isoforms with four repeats each, and its exclusion results in the production of a further three isoforms with three repeats each. The repeats and some adjoining sequences constitute the microtubule-binding domains of tau, with four-repeat tau being better at the promotion of microtubule assembly than three-repeat tau.37 Aminoacid residues Ser214–Glu372 of tau, corresponding to the region upstream of the repeats and the repeats themselves, bind tightly to microtubules.38 Structural biochemistry has provided information on the interaction between tau and microtubules. Microtubules were assembled with tubulin and tau in the presence of trimethylamine N-oxide and in the absence of paclitaxel.39 The repeats of tau bound to a region on the inner surface of the microtubule overlapping the paclitaxel-binding site of β-tubulin. This model supports the view that the proline-rich region of tau provides the link between its amino-terminal projection domain on the outside of the microtubule and its repeats on the inside of the microtubule. Experiments in which tau was bound to preassembled, paclitaxel-stabilised microtubules have given rise to a different model40 in which tau was bound to the outer surface of the microtubule, where it localised to the ridges of the protofilaments. Equal amounts of three-repeat and four-repeat tau are found in the cerebral cortex of healthy adults.37 The expression of tau is roughly two-times higher in grey matter of the neocortex than in white matter and in the cerebellum.41 Because the assembly of tau is concentration dependent, regional variation in expression of the protein could favour its assembly. Alternative mRNA splicing of MAPT is similar among brain regions. However, localised changes in the alternative mRNA splicing of MAPT might give rise to sporadic tauopathies. In the developing human brain, only the shortest tau isoform (ie, three-repeat tau with no amino-terminal inserts) is expressed. Although tau is expressed in many forms in vertebrates, the isoform ratios are not conserved. Tau isoforms with three, four, or five repeats are expressed in the brains of adult chickens, whereas most adult rodents express isoforms with four repeats.42,43 Similar repeats are present in the high-molecular-weight proteins MAP2 and MAP4. The genomes of Caenorhabditis elegans and Drosophila melanogaster each encode one protein with tau-like repeats. Four different tau knockout lines have been generated in mice.44–47 Animals from one line developed muscle weakness, motor deficits, hyperactivity, and learning difficulties.48 Mice from a second tau www.thelancet.com/neurology Vol 12 June 2013

Review

knockout line accumulated increased concentrations of iron in the substantia nigra, and had dopaminergic nerve cell loss, levodopa-responsive parkinsonism, and cognitive deficits.49 However, this phenotype was not reported in a separate study with the same tau knockout line.50 Upon examination, mice heterozygous for the MAPT deletion were unaffected, showing that a 50% reduction in the amount of tau is well tolerated.

Tau aggregation Tau is a natively unfolded protein that assembles into filaments through its tandem repeats, with the aminoterminal half and the carboxy-terminus forming the socalled fuzzy coat of the filament.15,16,51 Tau filaments have a cross-β structure characteristic of amyloid filaments.52 In Alzheimer’s disease, after tangle-bearing cells die, tau filaments can remain in the extracellular space as socalled ghost tangles consisting largely of the repeat region of tau. Unsurprisingly then, the formation of tau aggregates is deleterious, and experiments have shown that aggregates made of the repeat region of tau can induce neurotoxicity.53 In some human tauopathies, such as Pick’s disease, progressive supranuclear palsy, corticobasal degeneration, and most cases caused by MAPT mutations, filamentous tau does not accumulate in the extracellular space after the death of aggregatebearing nerve cells. The reasons underlying these differences remain to be established. Filaments can be assembled from non-phosphorylated full-length recombinant tau through interaction with negatively charged compounds, including sulphated glycosaminoglycans, RNA, and free fatty acids.54–57 Heparin binds to the repeats and induces the dimerisation of tau, with filaments growing through monomer addition.58 Sequences in the second (aminoacids 275–280, Val-Gln-Ile-Ile-Asn-Lys) and third (aminoacids 306–311, Val-Gln-Ile-Val-Tyr-Lys) repeats are essential for the heparin-induced assembly of tau into filaments.59 Evidence showing the relevance of tau aggregation in nerve cell dysfunction exists.60 However, additional work is needed in mouse models with abundant tau aggregates and nerve cell loss. The conformation of tau might influence fast anterograde, kinesin-dependent axonal transport. In squid axoplasm, unlike soluble tau, filaments had reduced transport through a mechanism involving PP1 and GSK-3β.61 In tau, aminoacids 2–18 activate this pathway, suggesting that disease-associated changes in the conformation of tau can increase exposure of the phosphatase-activating domain. Similarly, the presence of tau filaments was associated with an alteration in axonal transport in the optic nerve of mice transgenic for human mutant Pro301Ser tau.62

Tau phosphorylation In all diseases in which tau filaments are implicated, tau is hyperphosphorylated and, as a result, is unable to interact with microtubules.63,64 A healthy human brain www.thelancet.com/neurology Vol 12 June 2013

Panel: Diseases with tau inclusions • Alzheimer’s disease • Amyotrophic lateral sclerosis and parkinsonism-dementia complex • Argyrophilic grain disease • Chronic traumatic encephalopathy • Corticobasal degeneration • Diffuse neurofibrillary tangles with calcification • Down’s syndrome • Familial British dementia • Familial Danish dementia • Frontotemporal dementia and parkinsonism linked to chromosome 17 (caused by MAPT mutations) • Frontotemporal lobar degeneration (some cases caused by C9ORF72 mutations) • Gerstmann-Sträussler-Scheinker disease • Guadeloupean parkinsonism • Myotonic dystrophy • Neurodegeneration with brain iron accumulation • Niemann-Pick disease, type C • Non-Guamanian motor neuron disease with neurofibrillary tangles • Pick’s disease • Postencephalitic parkinsonism • Prion protein cerebral amyloid angiopathy • Progressive subcortical gliosis • Progressive supranuclear palsy • SLC9A6-related mental retardation • Subacute sclerosing panencephalitis • Tangle-only dementia • White matter tauopathy with globular glial inclusions

has, on average, 1·9 moles of phosphate per mole of tau, whereas tau from the abnormal filaments of patients with Alzheimer’s disease carries 6–8 moles of phosphate per mole of tau.65 Although some tau sites are more phosphorylated in the diseased than in the healthy brain, others are only phosphorylated in the diseased brain. Tau hyperphosphorylation seems to precede filament assembly, but whether this process is necessary or sufficient for filament assembly is unknown. Phosphorylation-dependent anti-tau antibodies have been invaluable in the study of the pathogenic role of tau in neurodegeneration.66 Many tau phosphorylation sites are known; so too are candidate protein kinases and phosphatases.11,67 Prolinedirected kinases, protein kinases that phosphorylate the Lys-X-Gly-Ser motifs in the repeats, some tyrosine kinases, and PP2A have been suggested to be involved in the phosphorylation and dephosphorylation of tau. Increased phosphorylation of tau is not necessarily detrimental because the process happens reversibly during fetal brain development, hibernation, and hypothermia.68–71 Hypothermia induces an exponential decrease in PP2A activity. Reductions in both the expression and activity of 611

Review

A Exons:

0

2 3

1

4

4a

5

6

1 1 1

7 8

9

R1

R2

R3

R4

R1

R2

R3

R4

R1

1 1 1

10 11 12 13

R2

R3

R4

R1

R3

R4

R1

R3

R4

R1

R3

R4

14

441 412

4 repeat

383

410 381

3 repeat

352

B –15 –10

+3 +4 +11 +12 +13 +14 +16

post-translational modifications have been described: tau acetylation,74–76 glycation,77,78 glycosylation with O-linked N-acetylglucosamine (O-GlcNAcylation), which is added to the hydroxyl groups of serines and threonines,79 nitration,80 ubiquitination,81,82 sumoylation,83 prolylisomerisation,84,85 and truncation.16,86 Acetylation impairs the ability of tau to bind to microtubules and increases its propensity to assemble into filaments; acetylation seems to occur after phosphorylation. An inverse relation appears to exist between the extent of O-GlcNAcylation and the degree of tau phosphorylation. Thus, modification of tau by O-GlcNAcylation causes a reduction in tau phosphorylation. Phosphorylated tau can exist in either the cis or trans forms. Some studies suggest that the cis but not the trans, form of pT231 tau is pathogenic in Alzheimer’s disease and that the prolyl isomerase Pin1 is protective because it accelerates the conversion from cis to trans. Nitration and ubiquitination of tau are late events. Truncation happens after filament assembly, when the fuzzy coat of the tau filament is cleaved off. Whether the truncation of tau can also be an early event remains to be seen.

Isoform composition of tau filaments E11

E12

E13

E372G G389R R406W N410H T427M

E10

N279K L284L, L284R ΔN296, N296N, N296H K298E P301L, P301S, P301T G303V G304S S305I, S305N, S305S L315R K317M S320F P332S G335S, G335V Q336R V337M E342V S352L S356T V363I P364S G366R K369I

E9

K257T I260V L266V G272V G273R

R5H, R5L

E1

Figure 2: Human brain tau isoforms and MAPT mutations (A) MAPT and the six tau isoforms expressed in an adult human brain. MAPT consists of 16 exons (E). Alternative mRNA splicing of E2 (red), E3 (light green), and E10 (orange) gives rise to the six tau isoforms (aminoacids 352– 441). The constitutively spliced exons (E1, E4, E5, E7, E9, E11, E12, and E13) are shown in blue. E0, which is part of the promoter, and E14 are non-coding (white). E6 and E8 (violet) are not transcribed in human brain. E4a (dark green) is expressed only in the peripheral nervous system. The repeats of tau (R1–R4) are shown, with three isoforms having four repeats each (4 repeat) and three isoforms having three repeats each (3 repeat). Each repeat is 31 or 32 aminoacids in length. The exons and introns are not drawn to scale. (B) Mutations in MAPT in cases of frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17T). 42 coding-region mutations in MAPT and nine intronic mutations flanking E10 are shown. Adapted from Goedert and colleagues,33 by permission of the Cold Spring Harbor Laboratory Press.

PP2A have also been reported in the brains of patients with Alzheimer’s disease.72,73 Whether different mechanisms underlie the physiological and pathological phosphorylation of tau remains to be seen. In particular, quantitative knowledge about the sites involved is scarce. In human diseases, tau might first misfold, rendering it a better substrate for protein kinases and a worse substrate for protein phosphatases. This change in conformation could result in hyperphosphorylation of tau and an increase in the amount of tau not bound to microtubules. Although phosphorylation of serine and threonine is a major early characteristic of tau aggregation, other 612

The presence of filamentous deposits of hyperphosphorylated tau in the human brain raises the question: why do several tauopathies exist rather than just one disease? An explanation of this finding might be that distinct brain regions and cell types are affected in different human tauopathies. These differences are partly related to the isoform composition of tau filaments. All six brain tau isoforms are present in tau filaments in the brains of patients with Alzheimer’s disease.19 Tau filaments in patients with progressive supranuclear palsy and corticobasal degeneration (both diseases of movement and cognition) and argyrophilic grain disease (a mainly dementing disease) are made of four-repeat tau, whereas tau filaments of patients with Pick’s disease (a form of frontotemporal dementia) are made of three-repeat tau.87–91 These differences are shown in the different morphologies of tau filaments.92 The filaments are reminiscent—on the basis of the existence of separate conformers of assembled proteins—of mammalian and yeast prions, of which different strains exist.93 Tau isoforms are characterised by an asymmetric seeding barrier, according to which seeds of three-repeat tau can recruit four-repeat tau; however, seeds of four-repeat tau cannot recruit three-repeat tau.94

Genetics of MAPT Genetic studies established the link between tau dysfunction and neurodegeneration. In 1994, a dominantly inherited form of frontotemporal dementia and parkinsonism was linked to chromosome 17q21–22,95 through a region containing MAPT. Additional forms of frontotemporal dementia were subsequently linked to the same region, leading to the denomination frontotemporal www.thelancet.com/neurology Vol 12 June 2013

Review

dementia and parkinsonism linked to chromosome 17 (FTDP-17) for this class of disease.96 Many patients with FTDP-17 have tau inclusions in nerve cells or in both nerve cells and glial cells. In 1998, the first mutations were reported in MAPT, in what is now called FTDP17T.21–23 As of April 2013, 51 pathogenic MAPT mutations have been identified (figure 2). However, not all cases of FTDP-17 are caused by mutations in MAPT. In 2006, the cause of FTDP-17 was discovered to be mutations in either MAPT or GRN.97,98 Mutations in MAPT account for about 5% of cases of frontotemporal dementia and seem to cause disease through a gain of toxic function.33 Most mutations are located in exons 9–12 (which encode the repeats) and the adjacent introns, and can be divided into two groups: those with a primary effect at the protein level and those that affect the alternative splicing of tau pre-mRNA. Mutations acting at the protein level change or delete single aminoacids, and thereby reduce the ability of tau to interact with microtubules. This partial loss of function might be needed for the abnormal aggregation of tau— ie, the gain of toxic function. Some mutations also promote the assembly of tau into filaments. Mutations with a primary effect at the RNA level are intronic or exonic and increase the alternative mRNA splicing of exon 10 of MAPT. This affects the ratio of three-repeat to four-repeat isoforms, resulting in the overproduction of four-repeat tau compared with three-repeat, and the assembly of four-repeat isoforms into filaments. Most missense mutations in MAPT do not result in the addition or removal of phosphorylation sites, suggesting that tau hyperphosphorylation is not the main event in FTDP-17T. However, these mutations might enable hyperphosphorylation of tau.99 Clinical and neuropathological phenotypes similar or identical to Pick’s disease, progressive supranuclear palsy, corticobasal degeneration, and argyrophilic grain disease have been described in FTDP-17T. A single mutation can also cause different syndromes in the same family. Thus, mutant Pro301Ser in exon 10 of MAPT gave rise to behaviouralvariant frontotemporal dementia in a father and corticobasal degeneration in his son, supporting the view that frontotemporal dementia and corticobasal degeneration form part of the same disease spectrum.100 MAPT in populations of European descent is characterised by two haplotypes that result from a 900-kb inversion (H1) or non-inversion (H2) polymorphism.101 Inheritance of the H1 haplotype is a risk factor for progressive supranuclear palsy, corticobasal degeneration, and idiopathic Parkinson’s disease.102–105 Subhaplotype H1c in a regulatory region in intron 0 of MAPT is associated with increased risk of progressive supranuclear palsy and corticobasal degeneration. This hypothesis has been confirmed in a genome-wide association study of progressive supranuclear palsy, which also suggested the involvement of proteins in vesicle trafficking (STX6), the structure of myelin (MOBP), and the unfolded protein response www.thelancet.com/neurology Vol 12 June 2013

(PERK).106 The association of the H1 MAPT haplotype with progressive supranuclear palsy had a higher odds ratio than the association between APOE4 and Alzheimer’s disease.106 APOE4 is the major risk factor allele for lateonset Alzheimer’s disease.107 The H2 haplotype is associated with increased expression of exon 3 of MAPT in grey matter, suggesting that the inclusion of exon 3 is protective against progressive supranuclear palsy, corticobasal degeneration, and Parkinson’s disease.108 Tau isoforms containing exons 2 and 10 promote tau aggregation, with exon 3-containing isoforms being inhibitory.109 Heterozygous microdeletions in chromosome 17q21.31 give rise to a multisystem disorder, which is characterised by intellectual disability, hypotonia, and distinctive facial features (17q21.31 microdeletion syndrome).110–112 In addition to MAPT, three known protein-coding genes (CRHR1, SPPL2C, and KANSL1) and two putative genes (MGC57346 and CRHR1-IT1) are found in this region. Deletions arise on the H2 haplotype through low-copy, repeat-mediated, non-allelic homologous recombination. The 17q21.31 microdeletion syndrome is caused by haploinsufficiency of KANSL1, which encodes a chromatin modifier that influences gene expression through the acetylation of lysine 16 of histone H4.113,114 A 50% reduction in tau levels does not have a detrimental effect on development of the human brain.

Aβ and tau Alzheimer’s disease is defined by the presence of abundant neuritic plaques and neurofibrillary lesions. Many disease-causing mutations in the genes encoding amyloid precursor protein (APP), and presenilin 1 (PSEN1) and presenilin 2 (PSEN2) result in the increased production of amyloid β 42 in the brain. Other mutations in APP promote the aggregation of amyloid β 42. These mutations and their effects form the backbone of the amyloid hypothesis of Alzheimer’s disease,115 which postulates that the accumulation of amyloid β leads to amyloid β oligomerisation, tau hyperphosphorylation and aggregation, synaptic dysfunction, nerve cell death, and brain shrinkage. Moreover, a heterozygous mutation in APP (Ala673Thr) that results in a 40% reduction in amyloid β protects against Alzheimer’s disease and agerelated cognitive decline.116 Although disease-causing mutations exert their effects throughout life, the symptoms of inherited Alzheimer’s disease tend to appear after the fourth decade of life, consistent with a preclinical phase, during which many biomarkers change years before symptoms appear. Reports from the Dominantly Inherited Alzheimer’s disease Network (DIAN)117 and studies of carriers of the Glu280Ala mutation in PSEN1118,119 have shown that the concentration of amyloid β 42 in CSF increased initially but then decreased decades before the expected onset of symptoms. Amyloid β deposits, measured with PET and Pittsburgh compound B, were detected 15 years before 613

Review

the onset of symptoms. Concentrations of tau in CSF increased 15 years before symptoms developed, with global cognitive impairment starting 5 years before manifest clinical symptoms. Mutation carriers with the Glu280Ala mutation had reduced grey matter volume and altered synaptic function many years before the onset of symptoms. Unless these changes were developmental, neurodegeneration might therefore start before amyloid β deposition. This conclusion is supported by findings in cognitively normal elderly individuals, in whom biomarkers of brain injury appeared independently of amyloid β deposition.120 Imaging methods for the specific detection of tau aggregates in the human brain are under development.121 Whether raised concentrations of tau in CSF relate to tau aggregation is unknown. Several lines of evidence suggest that the formation of amyloid β can drive tau pathology, even though tau deposits form in the brains of many people before amyloid β plaques. Tau inclusions have been suggested to form in most people as they age, and clinical Alzheimer’s disease has been proposed to arise only when amyloid β deposits have formed separately and driven the formation of tau inclusions in the neocortex.122 When mice transgenic for human mutant APP were crossed with mice transgenic for human mutant tau, tau pathology, but not amyloid β pathology, was exacerbated in the cerebral cortex.123 The intracerebral injection of synthetic amyloid β aggregates into mice transgenic for human mutant tau increased the number of tau inclusions.124 The formation of tau inclusions was similarly promoted by extracellular deposits of the Bri peptide.125 However, in these studies, the tau protein was mutant, unlike in familial Alzheimer’s disease and Danish dementia, two human diseases with extracellular deposits of form of amyloid (amyloid β or Bri peptide) and intraneuronal tau inclusions. Familial Danish dementia is caused by a dominantly inherited decamer duplication in the 3ȸ region of the integral membrane protein 2 (or BRI) gene.126 The duplication produces a frameshift that removes the stop codon and results in the deposition of a form of amyloid consisting of the 34 carboxy-terminal aminoacids of mutant BRI. Familial British dementia is caused by a dominantly inherited mutation in BRI that removes the stop codon and results in the deposition of the same 34 aminoacid peptide.127 Evidence shows that amyloid β oligomers contribute to synaptic dysfunction. Mice transgenic for human mutant APP show cognitive and behavioural deficits in the absence of substantial nerve cell loss.128 These deficits include the induction of long-term depression and the impairment of long-term potentiation. Amyloid β oligomers, PrPc, and Fyn are enriched in postsynaptic densities.129,130 The complex between Aβ and PrPc activates Fyn, which in turn phosphorylates the NR2B subunit of the NMDA receptor, resulting in reduced NMDA receptor surface expression and loss of dendritic spines. 614

Tyr18 in tau is also phosphorylated after exposure of cells to soluble amyloid β.131 Excitotoxic mechanisms and overactivity of some brain networks seem to be important in the coordination of the toxic effects of amyloid β.132 Tau is necessary for amyloid β-induced neurotoxicity,133–138 and the hyperexcitability of neurons, which happens after exposure to amyloid β, is reduced in the absence of tau. Additional receptors for oligomeric amyloid β might exist, and it remains to be seen whether other protein aggregates can activate the same pathway. In view of evidence that tau is essential for the transport of Fyn to dendritic spines, this pathway could provide a link between amyloid β and tau. A reduction in tau also increases resistance to ApoE4-induced neuronal and cognitive impairments.139 These effects seem to be mediated through normal tau and are independent of its somatodendritic localisation, hyperphosphorylation, and aggregation. Synaptic dysfunction has been described as an early change in mice transgenic for human mutant tau.140 Dysfunction was dependent on somatodendritic localisation and hyperphosphorylation of tau, which led to its accumulation in dendritic spines, where it suppressed synaptic responses mediated by AMPA.

Propagation of tau pathology Findings from studies investigating the appearance of amyloid β deposits and tau aggregates in the human brain as a function of age suggest that tau inclusions appear at a younger age than do amyloid β plaques.141,142 Sparse tau aggregation has been described in the brains of most individuals from the general population who are younger than 30 years of age. Misfolded, hyperphosphorylated, silver-stain-negative tau first accumulates in the locus coeruleus, from where the pathology spreads to the entorhinal cortex and other brain regions. This differential distribution underlies the so-called Braak stages of tau pathology, which range from 1 to 6. Stages 1 to 2 are associated with prodromal Alzheimer’s disease, stages 3 to 4 with mild cognitive impairment, and stages 5 to 6 with Alzheimer’s disease. Stages 1 and 2 show silver-stainpositive tau aggregation that is largely confined to the upper layers of the transentorhinal cortex. Stages 3 and 4 are characterised by the severe involvement of the transentorhinal and entorhinal regions, with a less severe involvement of the hippocampus and several subcortical nuclei. Stages 5 and 6 show the widespread formation of argyrophilic tau aggregates in neocortical association areas and a further increase in pathology in the brain areas affected during stages 1 to 4. Stereotypical temporospatial spreading of tau inclusions has also been described in argyrophilic grain disease,143 in which the earliest changes were restricted to the ambient gyrus (stage 1), from where the pathological process extended to the anterior and posterior medial temporal lobe (stage 2), followed by the septum, insular cortex, and anterior cingulate gyrus (stage 3). Stage 3 was characteristic of patients with dementia. www.thelancet.com/neurology Vol 12 June 2013

Review

The propagation of tauopathy can be shown experimentally. Brainstem extracts were taken from mice engineered to express the human mutant Pro301Ser tau protein (with silver-stain-positive inclusions). These extracts were injected into the brains of mice expressing human wild-type four-repeat tau (without silver-stain-positive inclusions), which induced the assembly of wild-type tau into silver-stain-positive inclusions and caused the pathology at the sites of injection to spread to neighbouring brain regions. Neurodegeneration was not recorded for up to 18 months after the injection of Pro301Ser tau brain homogenates.144 This finding is in contrast to those for the mouse line transgenic for human mutant Pro301Ser tau, in which massive neurodegeneration was seen by 5 months of age.145 The molecular tau species responsible for propagation might be different from the species responsible for toxicity. The induction of tau pathology was dependent on the presence of insoluble human Pro301Ser tau.144 In agreement, the injection of filaments assembled from recombinant human mutant tau into the brains of young mice transgenic for human mutant Pro301Ser tau (before tau aggregates were evident) accelerated the formation of tau inclusions,146,147 consistent with the view that filamentous tau is sufficient to convert soluble tau into aggregates. Monomeric tau has also been detected in brain interstitial fluid, suggesting that it is released from nerve cells despite the absence of a signal sequence.148 Concentrations of monomeric and aggregated tau were in equilibrium in the extracellular space, but their relative concentrations were inversely related. Secretion of nonaggregated tau has also been described from cultured neuronal cells.149,150 Extracellular tau was reported to be neurotoxic because of the increase in intracellular calcium concentrations after the activation of muscarinic cholinergic receptors.151 The presence of the exon 2 insert in tau isoforms inhibits secretion.152 A link might exist between the secretion of monomeric tau and increased concentrations of tau in CSF. The intercellular transfer of tau inclusions has been shown in cultured cells.153–158 Filaments made of recombinant tau and tau filaments from the brains of patients with Alzheimer’s disease were taken up by cells via macropinocytosis and induced the aggregation of cytoplasmic tau. The internalisation of aggregated tau depended on the presence of sulphated glycosaminoglycans at the cell surface. Aggregated tau was released directly into the extracellular space, but the underlying mechanisms remain to be identified. These findings were confirmed and extended by reports of the propagation of tau pathology from the entorhinal cortex to synaptically connected brain regions in transgenic mice.159,160 In bigenic mice that apparently only expressed human mutant Pro301Leu tau in parts of the entorhinal cortex and the subiculum, tau inclusions propagated in www.thelancet.com/neurology Vol 12 June 2013

an anterograde manner to the dentate gyrus and layers CA1 and CA3 of the hippocampus, consistent with their trans-synaptic spread.

Therapeutic implications Although most drug trials in Alzheimer’s disease so far have focused on amyloid β, interest in tau-targeted treatments is growing because of the discovery of the central role of tau in neurodegeneration and the difficulties associated with the targeting of amyloid β to develop mechanism-based treatments for Alzheimer’s disease.

Inhibition of tau aggregation The pathway leading from soluble and monomeric to hyperphosphorylated, insoluble, and filamentous tau is central to human tauopathies.11 Therefore, inhibition of aggregation and disassembly of tau aggregates are both promising therapeutic avenues. The repeat region forms the core of the tau filament15,16 and the third repeat is the most important for assembly of the filament.161 The aminoterminus of the third repeat of tau consists of the hexapeptide Val-Gln-Ile-Val-Tyr-Lys. On the basis of the atomic structure of this motif, an all d-aminoacid peptide was designed that inhibited tau filament formation.162 Small molecules can also inhibit tau filament formation in vitro—eg, inhibitory polyphenols, phenothiazines, anthraquinones, and quinoxalines.163–165 Grape seed polyphenolic extract attenuated the development of pathological changes in a mouse model of tauopathy.166 Phenothiazine methylene blue was the first compound reported to inhibit tau aggregation in vitro.167 The compound inhibits aggregation by modification of the cysteine residues in the repeats of tau (three-repeat tau has one cysteine, with two cysteines in four-repeat tau) to sulfenic, sulfinic, and sulfonic acid. As a result, tau is aggregation incompetent.168 In a mouse line transgenic for human mutant tau, methylene blue stimulated autophagy and reduced the concentrations of soluble tau and hyperphosphorylated sarkosyl-insoluble tau.169–171 In a phase 2 clinical trial for mild to moderate Alzheimer’s disease, methylene blue was effective in the treatment of cognitive deficits.172 Results from a phase 3 trial are needed to assess the therapeutic potential of methylene blue.

Inhibition of tau phosphorylation The hyperphosphorylation of tau seems to be an early and crucial event in tau-mediated neurodegeneration. The amount of phosphorylation depends on the conformation of tau and on the balance between the activities of tau kinases and tau phosphatases. Inhibition of tau kinases and activation of tau phosphatases are therapeutic targets for Alzheimer’s disease and other tauopathies. However, because these enzymes have several substrates, whether effective and safe tau kinase inhibitors and tau phosphatase activators can be developed remains to be seen. There are many sites that 615

Review

can be hyperphosphorylated in tau, suggesting the involvement of many protein kinases, although it seems probable that not all the phosphorylated sites have the same functional importance. In a mouse line transgenic for human mutant Pro301Leu tau, treatment with the non-specific protein kinase inhibitor K252a reduced the amount of soluble aggregated hyperphosphorylated tau and improved motor symptoms.173 Similar findings were reported when inhibitors of the kinase GSK-3 were used,174 consistent with a beneficial effect of protein kinase inhibition. Accumulating evidence suggests that GSK-3 provides a link between amyloid β and tau phosphorylation in patients with Alzheimer’s disease.175 However, a clear beneficial effect of long-term kinase inhibition on neurodegeneration remains to be shown. In patients with Alzheimer’s disease, a short-term trial of lithium chloride, an inhibitor of GSK-3, was not successful.176 The same was true when tideglusib (NP-12), a more specific GSK-3 inhibitor,177 was assessed in patients with Alzheimer’s disease.178 No clinical benefit was seen in patients with progressive supranuclear palsy treated with tideglusib; however, the drug substantially reduced the rate of brain atrophy. Another strategy is to activate the protein phosphatases that remove phosphate groups from tau. PP2A is the main tau phosphatase in brain.179 It is composed of a catalytic (C), a scaffolding (A), and a regulatory (B) subunit that assemble to form a heterotrimeric holoenzyme. In the brain, ACB55α is the most active isoform against phosphorylated tau.180,181 Methylation of the C-terminus of PP2Ac increases the assembly of active PP2A.182 Folic acid, which is found at reduced concentrations in patients with Alzheimer’s disease and contributes to increased concentrations of homocysteine, promotes methylation of PP2Ac.183 The antidiabetic drug metformin reduces tau phosphorylation through an increase in PP2A activity.184 Similarly, chronic low doses of sodium selenate increased PP2A activity and reduced tau phosphorylation in mouse models of tauopathy.185 Tau aggregation and neurodegeneration were decreased and cognition was improved. Tau is also modified by O-linked glycosylation.79 The amount of O-linked glycosylation of tau is decreased in brain extracts from patients with Alzheimer’s disease.186 Chronic treatment with thiamet G, a selective inhibitor of the enzyme that hydrolyses O-linked N-acetylglucosamine, decreased tau aggregation and increased neuronal survival in a mouse line transgenic for human mutant tau.187 However, these beneficial effects resulted from a reduction in the oligomerisation, not the phosphorylation, of tau.

Reduction of tau levels The aggregation of tau is energetically unfavourable and concentration dependent. Therefore, a reduction of tau concentrations is an attractive treatment approach that might be achieved through antisense or RNA interference 616

approaches, or through decreased tau expression188 and increased tau clearance. As described above, a partial reduction of tau in mice is well tolerated. To be fully effective, a reduction in tau concentrations will probably need to be achieved early, before the first tau aggregates form. This in turn raises the question of how to identify those who are in need of treatment. Soluble tau is degraded by the ubiquitin–proteasome system; the carboxy-terminus of heat shock protein 70-interacting protein (CHIP) and the molecular chaperone heat shock protein 90 have important roles.189–191 CHIP is a ligase that ubiquitinates denatured proteins when the substrate is captured by a molecular chaperone— eg, heat shock protein 90. CHIP binds to phosphorylated tau and is needed for tau ubiquitination and subsequent targeting of tau to the proteasome. USP14 is a proteasomeassociated deubiquitinating enzyme that inhibits the degradation of ubiquitin–protein conjugates. When coexpressed with USP14 in murine embryonic fibroblasts, the amount of soluble tau increased.192 Conversely, IU1, a small molecule inhibitor of USP14, stimulated the degradation of tau. These findings suggest that enhancement of proteasome function could be an effective therapeutic strategy for the tauopathies. An increase in the degradation of tau aggregates is a complementary approach. Aggregates are probably not accessible to the ubiquitin–proteasome system, but might be degraded by the autophagy–lysosome system. In a mouse line transgenic for human mutant tau and in a neuronal cell model of tauopathy, trehalose, which activates autophagy in an mTOR-independent manner,193 reduced the concentration of sarkosyl-insoluble tau and the number of nerve cells with tau inclusions, and substantially improved the survival of nerve cells.194,195

Tau immunisation If tau aggregates propagate between cells, they are likely to pass through an extracellular stage. The removal of extracellular tau aggregates by specific antibodies constitutes a potential mechanism-based treatment. Arresting the spread of tau aggregates at a preclinical stage might therefore prevent the appearance of clinical symptoms. In an in-vitro system, the propagation of tau aggregates was blocked by an anti-tau monoclonal antibody.156 Seven studies with either active or passive immunisation against tau have shown beneficial effects in transgenic mouse models of tauopathy.196–202 Active immunisation carries an inherent risk of causing tauopathy, implying that passive immunisation might be safer. Active immunisation strategies used tau phosphopeptides as antigens. By contrast, antibodies against phosphorylated or non-phosphorylated tau have been successfully used in studies of passive immunisation.

Anti-inflammatory treatments Microglial activation promotes the hyperphosphorylation of tau. Hyperphosphorylation is exacerbated in mice www.thelancet.com/neurology Vol 12 June 2013

Review

without the microglial-specific fractalkine receptor CX3CR1. Humanised MAPT transgenic mice without CX3CR1 had increased phosphorylation and aggregation of tau, which correlated with increased p38 MAP kinase activity.203 Fractalkine is produced by neurons and exerts an anti-inflammatory effect on microglia. Consequently, fractalkine overexpression substantially reduced the amount of tau pathology in a mouse model of human tauopathy.204 These findings suggest that the activation of fractalkine receptors is a valid therapeutic target for the tauopathies. Activation of microglial cells is believed to be an early event in mouse models of tauopathy.205,206 The immunosuppressant FK506 reduced microgliosis and tau aggregation in a mouse line transgenic for human mutant Pro301Ser tau. Whether the beneficial effects of FK506 on tau aggregation and neurodegeneration were the result of reduced glial activation is unknown. Inhibition of tau aggregation might also be achieved through the direct interaction between hyperphosphorylated tau and the immunophilin FKBP52.207

Search strategy and selection criteria We searched PubMed for articles published between Jan 1, 1975, and April 1, 2013, using the search terms: “tau protein”, “tauopathy”, “Alzheimer’s disease”, “tau aggregation”, “tau propagation”, and “MAPT mutations”. We searched for papers written in English, but relevant papers and books in French or German were also consulted. Articles were also obtained through searches of the authors’ own files. The authors attempted to achieve a balance between original studies and timely reviews.

neuronal precursor cells is beneficial in mouse models of tauopathy; these cells increased synaptic density, reduced nerve cell loss, and reversed cognitive deficits.216,217 The benefits were independent of alterations in tau pathology. The release of BDNF from the transplanted cells might have been beneficial.218 It follows that the deleterious effects of protein aggregates can be overcome, as has been reported for allopregnenolone219 and a peptide derived from ciliary-derived neurotrophic factor.220

Microtubule stabilisation Hyperphosphorylation of tau reduces its ability to interact with microtubules,63,64 suggesting a partial loss of function that might be necessary to start the gain of toxic function mechanisms believed to cause disease in humans. A partial loss of function of tau is unlikely to substantially impair microtubule function. Thus, heterozygous tau knockout mice have no phenotype44–47 and tau reduction protects neurons from amyloid β-induced impairments.133–138 Nonetheless, tau aggregation might destabilise microtubules, implying that microtubule stabilisation could be of therapeutic benefit. Hyperdynamic microtubules have been described in mouse lines that are transgenic for human mutant tau.208 Epothilone D, a microtubule stabiliser that can permeate the brain, increased the number of microtubules, reduced axonal dystrophy, improved axonal transport, and improved cognition in mouse lines transgenic for human mutant tau.209,210 Asn-Ala-Pro-Val-Ser-Ile-Pro-Gln (NAP), an octapeptide derived from a neuronal growth factor, represents a possible alternative to microtubule-stabilising compounds such as epothilone D. The octapeptide reduced tau pathology and enhanced cognitive function in a mouse model of human tauopathy.211 However, a recent study failed to show efficacy and safety of davunetide (NAP) for the treatment of patients with progressive supranuclear palsy.212

Cell replacement The mechanisms underlying tau-mediated nerve cell degeneration are incompletely understood. Caspase 3-mediated cleavage of tau has been reported to be necessary for tau assembly in one study,213 but not in others.214,215 Transplantation of neural stem cells or www.thelancet.com/neurology Vol 12 June 2013

Conclusions Biochemistry and immunostaining have shown that hyperphosphorylated, aggregated tau makes up the intracellular filamentous inclusions defining many human neurodegenerative diseases. Tau aggregation is associated with disease symptoms and is the likely mediator of neurodegeneration. Genetic studies have established that tau dysfunction is sufficient to cause neurodegeneration and dementia. Moreover, neuropathology and experimental studies have shown the relevance of cell non-autonomous mechanisms in the spread of tauopathy. A pathway leading from soluble, monomeric to insoluble, aggregated tau is at the centre of efforts to develop mechanism-based treatments for the tauopathies. Contributors Both authors contributed equally to researching data for the article, discussions of the content, writing the article, and review of the manuscript before submission. Conflicts of interest We declare that we have no conflicts of interest. This article was supported in part by the UK Medical Research Council (U105184291) and Alzheimer’s Research UK. References 1 Alzheimer A. On a peculiar disease of the cerebral cortex. Allg Z Psychiat 1907; 64: 146–48 (in German). 2 Fischer O. Miliary necroses with geodic proliferations of the neurofibrils, a regular change of the cerebral cortex in senile dementia. Monatsschr Psychiat Neurol 1907; 22: 361–72 (in German). 3 Goedert M, Ghetti B. Alois Alzheimer: his life and times. Brain Pathol 2007; 17: 57–62. 4 Goedert M. Oskar Fischer and the study of dementia. Brain 2009; 132: 1102–11. 5 Kraepelin E. Psychiatry: a textbook for students and medical doctors, vol 2, 8th edn. Leipzig: Barth, 1910 (in German). 6 Müller U, Winter P, Graeber MB. A presenilin 1 mutation in the first case of Alzheimer’s disease. Lancet Neurol 2013; 12: 129–30. 7 Katzman R. The prevalence and malignancy of Alzheimer disease. Arch Neurol 1976; 33: 217–18.

617

Review

8 9

10 11 12

13

14

15

16

17 18

19

20

21

22

23

24

25 26 27

28

29

30

31 32

618

Alzheimer A. On peculiar disease cases of later age. Z Ges Neurol Psychiat 1911; 4: 356–85 (in German). Lewy FH. Paralysis agitans. I. Pathological anatomy. In: Lewandowsky M, Abelsdorff G, eds. Handbook of neurology, vol 3. Berlin: Springer Verlag, Berlin, 1912: 920–33 (in German). Goedert M, Spillantini MG, Del Tredici K, Braak H. 100 years of Lewy pathology. Nat Rev Neurol 2013; 9: 13–24. Goedert M, Spillantini MG. A century of Alzheimer’s disease. Science 2006; 314: 777–81. Omalu B, Hammers JL, Bailes J, et al. Chronic traumatic encephalopathy in an Iraqi war veteran with posttraumatic stress disorder who committed suicide. Neurosurg Focus 2011; 31: 1–10. Brion JP, Passareiro H, Nunez J, Flament-Durand J. Immunological detection of tau protein in the lesions of the neurofibrillary degeneration of Alzheimer’s disease. Arch Biol (Bruxelles) 1985; 95: 229–35 (in French). Grundke-Iqbal I, Iqbal K, Tung YC, Quinlan M, Wisniewski HM, Binder LI. Abnormal phosphorylation of the microtubule-associated protein tau in Alzheimer cytoskeletal pathology. Proc Natl Acad Sci USA 1986; 83: 4913–17. Goedert M, Wischik CM, Crowther RA, Walker JE, Klug A. Cloning and sequencing of the cDNA encoding a core protein of the paired helical filament of Alzheimer disease: identification as the microtubule-associated protein tau. Proc Natl Acad Sci USA 1988; 85: 4051–55. Wischik CM, Novak M, Thogersen HC, et al. Isolation of a fragment of tau derived from the core of the paired helical filament of Alzheimer disease. Proc Natl Acad Sci USA 1988; 85: 4506–10. Kondo J, Honda T, Mori H, et al. The carboxyl third of tau is tightly bound to paired helical filaments. Neuron 1988; 1: 827–34. Lee VMY, Balin BJ, Otvos L, Trojanowski JQ. A68: a major subunit of paired helical filaments and derivatized forms of normal tau. Science 1991; 251: 675–78. Goedert M, Spillantini MG, Cairns NJ, Crowther RA. Tau proteins of Alzheimer paired helical filaments: Abnormal phosphorylation of all six brain isoforms. Neuron 1992; 8: 159–68. Greenberg SG, Davies P, Schein JD, Binder LI. Hydrofluoric acid-treated TauPHF proteins display the same biochemical properties as normal tau. J Biol Chem 1992; 267: 564–69. Poorkaj P, Bird TD, Wijsman E, et al. Tau is a candidate gene for chromosome 17 frontotemporal dementia. Ann Neurol 1998; 43: 815–25. Hutton M, Lendon CL, Rizzu M, et al. Association of missense and 5’-splice-site mutations in tau with the inherited dementia FTDP-17. Nature 1998; 393: 702–05. Spillantini MG, Murrell JR, Goedert M, Farlow MR, Klug A, Ghetti B. Mutation in the tau gene in familial multiple system tauopathy with presenile dementia. Proc Natl Acad Sci USA 1998; 95: 7737–41. McKee AC, Cantu RC, Nowinski CJ, et al. Chronic traumatic encephalopathy in athletes: progressive tauopathy after repetitive head injury. J Neuropathol Exp Neurol 2009; 68: 709–35. McKee AC, Stein TD, Nowinski CJ, et al. The spectrum of disease in chronic traumatic encephalopathy. Brain 2013; 136: 43–64. Martland HS. Punch drunk. JAMA 1928; 91: 1103–07. Glenner GG, Wong CW. Alzheimer’s disease: initial report on the purification and characterization of a novel cerebrovascular amyloid protein. Biochem Biophys Res Commun 1984; 120: 885–90. Masters CL, Simms G, Weinman NA, Multhaup G, McDonald BL, Beyreuther K. Amyloid plaque core protein in Alzheimer disease and Down syndrome. Proc Natl Acad Sci USA 1985; 82: 4245–49. Revesz T, Holton JL, Doshi B, Anderton BH, Scaravilli F, Plant GT. Cytoskeletal pathology in familial cerebral amyloid angiopathy (British type) with non-neuritic amyloid plaque formation. Acta Neuropathol 1999; 97: 170–76. Ghetti B, Tagliavini F, Masters CL, et al. Gerstmann-SträusslerScheinker disease: 2. Neurofibrillary tangles and plaques with PrP-amyloid coexist in an affected family. Neurology 1989; 39: 1453–61. Binder LI, Frankfurter A, Rebhun KI. The distribution of tau in the mammalian central nervous system. J Cell Biol 1985; 101: 1371–78. Ittner LM, Ke YD, Delerue F, et al. Dendritic function of tau mediates amyloid-beta toxicity in Alzheimer’s disease mouse models. Cell 2010; 142: 387–97.

33

34

35

36 37

38

39

40

41

42

43

44

45 46

47

48

49

50

51

52

53

54

Goedert M, Ghetti B, Spillantini MG. Frontotemporal dementia: Implications for understanding Alzheimer disease. Cold Spring Harb Perspect Med 2012; 2: a006254. Neve RL, Harris P, Kosik KS, Kurnit DM, Donlon TA. Identification of cDNA clones for the human microtubule-associated protein tau and chromosomal localization of the genes for tau and microtubule-associated protein 2. Brain Res 1986; 387: 271–80. Goedert M, Spillantini MG, Jakes R, Rutherford D, Crowther RA. Multiple isoforms of human microtubule-associated protein tau: Sequences and localization in neurofibrillary tangles of Alzheimer’s disease. Neuron 1989; 3: 519–26. Andreadis A, Brown MW, Kosik KS. Structure and novel exons of the human tau gene. Biochemistry 1992; 31: 10626–33. Goedert M, Jakes R. Expression of separate isoforms of human tau protein: correlation with the tau pattern in brain and effects on tubulin polymerization. EMBO J 1990; 9: 4225–30. Fauquant C, Redeker V, Landrieu I, et al. Systematic identification of tubulin-interacting fragments of the microtubule-associated protein tau leads to a highly efficient promoter of microtubule assembly. J Biol Chem 2011; 286: 33358–68. Kar S, Fan J, Smith MJ, Goedert M, Amos LA. Repeat motifs of tau bind to the insides of microtubules in the absence of taxol. EMBO J 2003; 22: 70–77. Al-Bassam J, Ozer RS, Safer D, Halpain S, Milligan RA. MAP2 and tau bind longitudinally along the outer ridges of microtubule protofilaments. J Cell Biol 2002; 157: 1187–96. Trabzuni D, Wray S, Vandrovcova J, et al. MAPT expression and splicing is differentially regulated by brain region: relation to genotype and implication for tauopathies. Hum Mol Genet 2012; 21: 4094–4103. Götz J, Probst A, Spillantini MG, et al. Somatodendritic localisation and hyperphosphorylation of tau protein in transgenic mice expressing the longest human brain tau isoform. EMBO J 1995; 14: 1304–13. Yoshida H, Goedert M. Molecular cloning and functional characterization of chicken brain tau: Isoforms with up to five tandem repeats. Biochemistry 2002; 41: 15203–11. Harada A, Oguchi K, Okabe S, et al. Altered microtubule organization in small-calibre axons of mice lacking tau protein. Nature 1994; 369: 488–91. Tucker KL, Meyer M, Barde YA. Neurotrophins are required for nerve growth during development. Nat Neurosci 2001; 4: 29–37. Dawson HN, Ferreira A, Eyster MV, Ghoshal N, Binder LI, Vitek MP. Inhibition of neuronal maturation in primary hippocampal neurons from tau-deficient mice. J Cell Sci 2001; 114: 1179–87. Muramatsu K, Hashimoto Y, Uemura T, et al. Neuron-specific recombination by Cre recombinase inserted into the murine tau locus. Biochem Biophys Res Commun 2008; 370: 419–23. Ikegami S, Harada A, Hirokawa N. Muscle weakness, hyperactivity, and impairment in fear conditioning in tau-deficient mice. Neurosci Lett 2000; 279: 129–32. Lei P, Ayton S, Finkelstein DI, et al. Tau deficiency induces parkinsonism with dementia by impairing APP-mediated iron export. Nat Med 2012; 18: 291–95. Morris M, Hamto P, Adame A, Devidze N, Masliah E, Mucke L. Age-appropriate cognition and subtle dopamine-independent motor deficits in aged Tau knockout mice. Neurobiol Aging 2013; 4: 1523–29. Wegmann S, Medalsy ID, Mandelkow E, Müller DJ. The fuzzy coat of pathological human tau fibrils is a two-layered polyelectrolyte brush. Proc Natl Acad Sci USA 2013; 110: 313–21. Berriman J, Serpell LC, Oberg KA, Fink AL, Goedert M, Crowther RA. Tau filaments from human brain and from in vitro assembly of recombinant protein show cross-β structure. Proc Natl Acad Sci USA 2003; 100: 9034–38. Khlistunova I, Biernat J, Wang Y, et al. Inducible expression of tau repeat domain in cell models of tauopathy. Aggregation is toxic to cells but can be reversed by inhibitor drugs. J Biol Chem 2006; 281: 1205–14. Goedert M, Jakes R, Spillantini MG, Hasegawa M, Smith MJ, Crowther RA. Assembly of microtubule-associated protein tau into Alzheimer-like filaments induced by sulphated glycosaminoglycans. Nature 1996; 383: 550–53.

www.thelancet.com/neurology Vol 12 June 2013

Review

55

56

57 58

59

60

61

62

63

64

65

66

67

68

69

70

71

72

73

74 75

76

Pérez M, Valpuesta JM, Medina M, Montejo de Garcini E, Avila J. Polymerization of tau into filaments in the presence of heparin: the minimal sequence required for tau-tau interaction. J Neurochem 1996; 67: 1183–90. Kampers T, Friedhoff P, Biernat J, Mandelkow EM, Mandelkow E. RNA stimulates aggregation of microtubule-associated protein tau into Alzheimer-like paired helical filaments. FEBS Lett 1996; 399: 344–49. Wilson DM, Binder LI. Free fatty acids stimulate the polymerization of tau and amyloid beta peptides. Am J Pathol 1997; 150: 2181–95. Ramachandran G, Udgaonkar JB. Understanding the kinetic roles of the inducer heparin and of rod-like protofibrils during amyloid fibril formation by tau protein. J Biol Chem 2011; 286: 38948–59. von Bergen M, Friedhoff P, Biernat J, Heberle J, Mandelkow EM, Mandelkow E. Assembly of tau protein into Alzheimer paired helical filaments depends on a local sequence motif [(306)VQIVYK(311)] forming beta structure. Proc Natl Acad Sci USA 2000; 97: 5129–34. van der Jeugd A, Hochgräfe K, Ahmed T, et al. Cognitive defects are reversible in inducible mice expressing pro-aggregant full-length human tau. Acta Neuropathol 2012; 123: 787–805. Kanaan NM, Morfini GA, LaPointe NE, et al. Pathogenic forms of tau inhibit kinesin-dependent axonal transport through a mechanism involving activation of axonal phosphotransferases. J Neurosci 2011; 31: 9858–68. Bull ND, Giudi A, Goedert M, Martin KR, Spillantini MG. Reduced axonal transport and increased excitotoxic retinal ganglion cell degeneration in mice transgenic for human mutant P301S tau. PLoS One 2012; 7: e34724. Bramblett GT, Goedert M, Jakes R, Merrick SE, Trojanowski JQ, Lee VMY. Abnormal tau phosphorylation at Ser396 in Alzheimer’s disease recapitulates development and contributes to reduced microtubule binding. Neuron 1993; 10: 1089–99. Yoshida H, Ihara Y. Tau in paired helical filaments is functionally distinct from fetal tau: assembly incompetence of paired helical filament-tau. J Neurochem 1993; 61: 1183–86. Ksiezak-Reding H, Liu WK, Yen SH. Phosphate analysis and dephosphorylation of modified tau associated with paired helical filaments. Brain Res 1992; 597: 209–19. Mercken M, Vandermeeren M, Lübke U, et al. Monoclonal antibodies with selective specificity for Alzheimer tau are directed against phosphatase-sensitive epitopes. Acta Neuropathol 1992; 84: 265–72. Hanger DP, Anderton BH, Noble W. Tau phosphorylation: the therapeutic challenge for neurodegenerative disease. Trends Mol Med 2009; 15: 112–19. Goedert M, Jakes R, Crowther RA, et al. The abnormal phosphorylation of tau protein at Ser202 in Alzheimer disease recapitulates phosphorylation during development. Proc Natl Acad Sci USA 1993; 90: 5066–70. Arendt T, Stieler J, Strijkstra AM, et al. Reversible paired helical filament-like phosphorylation of tau is an adaptive process associated with neuronal plasticity in hibernating animals. J Neurosci 2003: 23: 6972–81. Stieler J, Bullmann T, Kohl F, et al. The physiological link between metabolic rate depression and tau phosphorylation in mammalian hibernation. PLoS One 2011; 6: e14530. Planel E, Miyasaka T, Launey T, et al. Alterations in glucose metabolism induce hypothermia leading to tau hyperphosphorylation through differential inhibition of kinase and phosphatase activities: Implications for Alzheimer’s disease. J Neurosci 2004; 24: 2401–11. Gong CX, Singh TJ, Grundke-Iqbal I, Iqbal K. Phosphoprotein phosphatase activities in Alzheimer disease brain. J Neurochem 1993: 61: 921–27. Vogelsberg-Ragaglia V, Schuck T, Trojanowski JQ, Lee VMY. PP2A mRNA expression is quantitatively decreased in Alzheimer’s disease hippocampus. Exp Neurol 2001; 168: 402–12. Min SW, Cho SH, Zhou Y, et al. Acetylation of tau inhibits its degradation and contributes to tauopathy. Neuron 2010; 67: 953–66. Cohen TJ, Guo JL, Hurtado DE, et al. The acetylation of tau inhibits its function and promotes pathological tau aggregation. Nat Commun 2011; 2: 252. Irwin DJ, Cohen TJ, Grossman M, et al. Acetylated tau, a novel pathological signature in Alzheimer’s disease and other tauopathies. Brain 2012; 135: 807–18.

www.thelancet.com/neurology Vol 12 June 2013

77

Smith MA, Taneda S, Richey PL, et al. Advanced Maillard reaction end products are associated with Alzheimer disease pathology. Proc Natl Acad Sci USA 1994; 91: 5710–14. 78 Ledesma MD, Bonay P, Colaco C, Avila J. Analysis of microtubule-associated protein tau glycation in paired helical filaments. J Biol Chem 1994; 269: 21614–19. 79 Arnold CS, Johnson GVW, Cole RN, Dong DLY, Lee M, Hart GW. The microtubule-associated protein tau is extensively modified with O-linked N-acetylglucosamine. J Biol Chem 1996; 271: 28741–44. 80 Reynolds MR, Reyes JF, Fu Y, et al. Tau nitration occurs at tyrosine 29 in the fibrillar lesions of Alzheimer’s disease and other tauopathies. J Neurosci 2006; 26: 10636–45. 81 Mori H, Kondo J, Ihara Y. Ubiquitin is a component of paired helical filaments in Alzheimer’s disease. Science 1987; 235: 1641–44. 82 Cripps D, Thomas SN, Jeng Y, Yang F, Davies P, Yang AJ. Alzheimer disease-specific conformation of hyperphosphorylated paired helical filament-tau is polyubiquitinated through Lys-48, Lys-11, and Lys-6 ubiquitin conjugation. J Biol Chem 2006; 281: 10825–38. 83 Dorval V, Fraser PE. Small ubiquitin-like modifier (SUMO) modification of natively unfolded proteins tau and α-synuclein. J Biol Chem 2006; 281: 9919–24. 84 Lu PJ, Wulf G, Zhou XZ, Davies P, Lu KP. The prolyl isomerase Pin1 restores the function of Alzheimer-associated phosphorylated tau protein. Nature 1999; 399: 784–88. 85 Nakamura K, Greenwood A, Binder l, et al. Proline isomer-specific antibodies reveal the early pathogenic tau conformation in Alzheimer’s disease. Cell 2012; 149: 232–44. 86 Gamblin TC, Chen F, Zambrano A, et al. Caspase cleavage of tau: linking amyloid and neurofibrillary tangles in Alzheimer disease. Proc Natl Acad Sci USA 2003; 100: 10032–37. 87 Flament S, Delacourte A, Verny M, Hauw JJ, Javoy-Agid F. Abnormal tau proteins in progressive supranuclear palsy. Similarities and differences with the neurofibrillary degeneration of the Alzheimer type. Acta Neuropathol 1991; 81: 591–96. 88 Spillantini MG, Goedert M, Crowther RA, Murrell JR, Farlow MR, Ghetti B. Familial multiple system tauopathy with presenile dementia: a disease with abundant neuronal and glial tau filaments. Proc Natl Acad Sci USA 1997; 94: 4113–18. 89 Ksiezak-Reding H, Morgan K, Mattiace LA, et al. Ultrastructure and biochemical composition of paired helical filaments in corticobasal degeneration. Am J Pathol 1994; 145: 1496–1508. 90 Tolnay M, Sergeant N, Ghestem A, et al. Argyrophilic grain disease and Alzheimer’s disease are distinguished by their different distribution of tau protein isoforms. Acta Neuropathol 2002; 104: 425–34. 91 Delacourte A, Robitaille Y, Sergeant N, et al. Specific pathological tau protein variants characterize Pick’s disease. J Neuropathol Exp Neurol 1996; 55: 159–68. 92 Crowther RA, Goedert M. Abnormal tau-containing filaments in neurodegenerative diseases. J Struct Biol 2000; 130: 271–79. 93 Colby DW, Prusiner SB. Prions. Cold Spring Harb Perspect Biol 2011; 3: a006833. 94 Dinkel PD, Siddiqua A, Huynh H, et al. Variations in filament conformation dictate seeding barrier between three- and four-repeat tau. Biochemistry 2011; 50: 4330–36. 95 Wilhelmsen KC, Lynch T, Pavlou E, Higgins M, Nygaard TG. Localization of disinhibition-dementia-parkinsonism-amyotrophy complex to 17q21-22. Am J Hum Genet 1994; 55: 1159–65. 96 Foster NL, Wilhelmsen K, Sima AA, et al. Frontotemporal dementia and parkinsonism linked to chromosome 17: a consensus conference. Ann Neurol 1997; 41: 706–15. 97 Baker M, Mackenzie IR, Pickering-Brown SM, et al. Mutations in progranulin cause tau-negative frontotemporal dementia linked to chromosome 17. Nature 2006; 442: 916–19. 98 Cruts M, Gijselinck I, Van der Zee J, et al. Null mutations in progranulin cause ubiquitin-positive frontotemporal dementia linked to chromosome 17q21. Nature 2006; 442: 920–24. 99 del Alonso AC, Mederlyova A, Novak M, Grundke-Iqbal I, Iqbal K. Promotion of hyperphosphorylation by frontotemporal dementia tau mutations. J Biol Chem 2004; 279: 34873–881. 100 Bugiani O, Murrell JR, Giaccone G, et al. Frontotemporal dementia and corticobasal degeneration in a family with a P301S mutation in Tau. J Neuropathol Exp Neurol 1999; 58: 667–77.

619

Review

101 Stefansson H, Helgason A, Thorleifsson G, et al. A common inversion under selection in Europeans. Nat Genet 2005; 37: 129–37. 102 Conrad C, Andreadis A, Trojanowski JQ, et al. Genetic evidence for the involvement of tau in progressive supranuclear palsy. Ann Neurol 1997; 41: 277–81. 103 Baker M, Litvan I, Houlden H, et al. Association of an extended haplotype in the tau gene with progressive supranuclear palsy. Hum Mol Genet 1999; 8: 711–15. 104 Houlden H, Baker M, Morris HR, et al. Corticobasal degeneration and progressive supranuclear palsy share a common tau haplotype. Neurology 2001; 56: 1702–06. 105 Pastor P, Ezquerra M, Munoz E, et al. Significant association between the tau gene A0/A0 genotype and Parkinson’s disease. Ann Neurol 2000; 47: 242–45. 106 Höglinger GU, Melhem NM, Dickson DW, et al. Identification of common variants influencing risk of the tauopathy progressive supranuclear palsy. Nat Genet 2011; 43: 699–705. 107 Corder EH, Saunders AM, Strittmatter WJ, et al. Gene dose of apolipoprotein E type 4 allele and the risk of Alzheimer’s disease in late onset families. Science 1993; 261: 921–23. 108 Caffrey TM, Joachim C, Wade-Martins R. Haplotype-specific expression of the N-terminal exons 2 and 3 at the human MAPT locus. Neurobiol Aging 2008; 29: 1923–29. 109 Zhong Q, Congdon EE, Nagaraja HN, Kuret J. Tau isoform composition influences rate and extent of filament formation. J Biol Chem 2012; 287: 20711–19. 110 Koolen DA, Vissers LELM, Pfundt R, et al. A new chromosome 17q21.31 microdeletion syndrome associated with a common inversion polymorphism. Nat Genet 2006; 38: 999–1001. 111 Sharp AJ, Hansen S, Selzer RR, et al. Discovery of previously unidentified genomic disorders from the duplication architecture of the human genome. Nat Genet 2006; 68: 812–14. 112 Shaw-Smith C, Pittman AM, Willatt L, et al. Microdeletion encompassing MAPT at chromosome 17q21.3 is associated with developmental delay and learning disability. Nat Genet 2006; 38: 1032–37. 113 Zollino M, Orteschi D, Murdolo M, et al. Mutations in KANSL1 cause the 17q21.31 microdeletion syndrome phenotype. Nat Genet 2012; 44: 636–38. 114 Koolen DA, Kramer JM, Neveling K, et al. Mutations in the chromatin modifier gene KANSL1 cause the 17q21.31 microdeletion syndrome. Nat Genet 2012; 44: 639–41. 115 Hardy J, Selkoe DJ. The amyloid hypothesis of Alzheimer’s disease: progress and problems on the road to therapeutics. Science 2002; 297: 353–56. 116 Jonsson T, Atwal JK, Steinberg S, et al. A mutation in APP protects against Alzheimer’s disease and age-related cognitive decline. Nature 2012; 488: 96–99. 117 Bateman RJ, Xiong C, Benzinger TLS, et al. Clinical and biomarker changes in dominantly inherited Alzheimer’s disease. N Engl J Med 2012; 367: 795–804. 118 Reiman EM, Quiroz YT, Fleisher AS, et al. Brain imaging and fluid biomarker analysis in young adults at genetic risk for autosomal dominant Alzheimer’s disease in the presenilin 1 E280A kindred: a case-control study. Lancet Neurol 2012; 11: 1048–56. 119 Fleisher AS, Chen K, Quiroz YT, et al. Florbetapir PET analysis of amyloid-β deposition in the presenilin 1 E280A autosomal dominant Alzheimer’s disease kindred: a cross-sectional study. Lancet Neurol 2012; 11: 1057–65. 120 Knopman DS, Jack CR, Wiste HJ, et al. Neuronal injury biomarkers are not dependent on β-amyloid in normal elderly. Ann Neurol 2012; published online Nov 23. DOI: 10.1002/ana.23816. 121 Chien BD, Bahri S, Szardenings AK, et al. Early clinical PET imaging results with the novel PHF-tau radioligand [F-18]-T807. J Alzheimers Dis 2013; 34: 457–68. 122 Price JL, Morris JC. Tangles and plaques in nondemented aging and “preclinical” Alzheimer’s disease. Ann Neurol 1999; 45: 358–68. 123 Lewis J, Dickson DW, Lin WL, et al. Enhanced neurofibrillary degeneration in transgenic mice expressing mutant tau and APP. Science 2001; 293: 1487–91. 124 Bolmont T, Clavaguera F, Meyer-Luehmann M, et al. Induction of tau pathology by intracerebral infusion of amyloid-beta-containing brain extract and by amyloid-beta deposition in APP × Tau transgenic mice. Am J Pathol 2007; 171: 2012–20.

620

125 Coomaraswamy J, Kilger E, Wölfing H, et al. Modeling familial Danish dementia in mice supports the concept of the amyloid hypothesis of Alzheimer’s disease. Proc Natl Acad Sci USA 2010; 107: 7969–74. 126 Vidal R, Revesz T, Rostagno A, et al. A decamer duplication in the 3’ region of the BRI gene originates an amyloid peptide that is associated with dementia in a Danish kindred. Proc Natl Acad Sci USA 2000; 97: 4920–25. 127 Vidal R, Frangione B, Rostagno A, et al. A stop-codon mutation in the BRI gene associated with familial British dementia. Nature 1999; 399: 776–81. 128 Huang Y, Mucke L. Alzheimer mechanisms and therapeutic strategies. Cell 2012; 148: 1204–22. 129 Laurén J, Gimbel DA, Nygaard HB, Gilbert JW, Strittmatter SM. Cellular prion protein mediates impairment of synaptic plasticity by amyloid-beta oligomers. Nature 2009; 457: 1128–32. 130 Um JW, Nygaard HK, Heiss JK, et al. Alzheimer amyloid-β oligomer bound to postsynaptic prion protein activates Fyn to impair neurons. Nat Neurosci 2012; 15: 1227–35. 131 Larson M, Sherman MW, Amar F, et al.The complex PrPc-Fyn couples human oligomeric Aβ with pathological tau changes in Alzheimer’s disease. J Neurosci 2012; 32: 16857–71. 132 Busche MA, Chen X, Henning HA, et al. Critical role of soluble amyloid-β for early hippocampal hyperactivity in a mouse model of Alzheimer’s disease. Proc Natl Acad Sci USA 2012; 109: 8740–45. 133 Rapoport M, Dawson HN, Binder LI, Vitek MP, Ferreira A. Tau is essential to β-amyloid-induced neurotoxicity. Proc Natl Acad Sci USA 2002; 99: 6364–69. 134 Roberson ED, Scearce-Levie K, Palop JJ, et al. Reducing endogenous tau ameliorates amyloid β-induced deficits in an Alzheimer’s disease model. Science 2007; 316: 750–54. 135 Vossel KA, Zhang K, Brodbeck J, et al. Tau reduction prevents Aβ-induced defects in axonal transport. Science 210; 330: 198. 136 Shipton OA, Leitz JR, Dworzak J, et al. Tau protein is required for amyloid β-induced impairment of hippocampal long-term potentiation. J Neurosci 2011; 31: 1688–92. 137 Roberson ED, Halabisky B, Yoo JW, et al. Amyloid-β/Fyn-induced synaptic, network, and cognitive impairments depend on tau levels in multiple mouse models of Alzheimer’s disease. J Neurosci 2012; 31: 700–11. 138 Nussbaum J, Schilling S, Cynis H, et al. Prion-like properties and tau-dependent cytotoxicity of pyroglutamylated amyloid-β. Nature 2012; 485: 651–55. 139 Andrews-Zwilling Y, Bien-Ly N, Xu Q, et al. Apolipoprotein E4 causes age- and tau-dependent impairment of GABAergic interneurons, leading to learning and memory deficits in mice. J Neurosci 2010; 30: 13707–17. 140 Hoover BR, Reed MN, Su J, et al. Tau mislocalization to dendritic spines mediates synaptic dysfunction independently of neurodegeneration. Neuron 2010; 68: 1067–81. 141 Braak H, Braak E. Neuropathological stageing of Alzheimer-related changes. Acta Neuropathol 1991; 82: 239–59. 142 Braak H, Del Tredici K. The pathological process underlying Alzheimer’s disease in individuals under thirty. Acta Neuropathol 2011; 121: 171–81. 143 Saito Y, Ruberu NN, Sawabe M, et al. Staging of argyrophilic grains: an age-associated tauopathy. J Neuropathol Exp Neurol 2004; 63: 911–18. 144 Clavaguera F, Bolmont T, Crowther RA, et al. Transmission and spreading of tauopathy in transgenic mouse brain. Nat Cell Biol 2009; 11: 909–13. 145 Allen B, Ingram E, Takao M, et al. Abundant tau filaments and nonapoptotic neurodegeneration in transgenic mice expressing human P301S tau protein. J Neurosci 2002; 22: 9340–51. 146 Iba M, Guo JL, McBride JD, Zhang B, Trojanowski JQ, Lee VMY. Synthetic tau fibrils mediate transmission of neurofibrillary tangles in a transgenic mouse model of Alzheimer’s-like tauopathy. J Neurosci 2013; 33: 1024–37. 147 Clavaguera F, Lavenir I, Falcon B, Frank S, Goedert M, Tolnay M. Prion-like templated misfolding in tauopathies. Brain Pathol 2013; 23: 342–49. 148 Yamada K, Cirrito JR, Stewart FR, et al. In vivo microdialysis reveals age-dependent decrease of brain interstitial fluid tau levels in P301S human tau transgenic mice. J Neurosci 2011; 31: 13110–17.

www.thelancet.com/neurology Vol 12 June 2013

Review

149 Saman S, Kim W, Raya M, et al. Exosome-associated tau is secreted in tauopathy models and is selectively phosphorylated in cerebrospinal fluid in early Alzheimer disease. J Biol Chem 2012; 287: 3842–49. 150 Karch CM, Jeng AT, Goate AM. Extracellular tau levels are influenced by variability in tau that is associated with tauopathies. J Biol Chem 2012; 287: 42751–62. 151 Gomez-Ramos A, Diaz-Hernandez M, Cuadros R, Hernandez F, Avila J. Extracellular tau is toxic to neuronal cells. FEBS Lett 2006; 580: 4842–50. 152 Kim W, Lee S, Hall GF. Secretion of human tau fragments resembling CSF-tau in Alzheimer’s disease is modulated by the presence of the exon 2 insert. FEBS Lett 2010; 584: 3085–88. 153 Frost B, Jacks RL, Diamond MI. Propagation of tau misfolding from the outside to the inside of a cell. J Biol Chem 2009; 284: 12845–52. 154 Guo JL, Lee VMY. Seeding of normal tau by pathological tau conformers drives pathogenesis of Alzheimer-like tangles. J Biol Chem 2011; 286: 15317–31. 155 Santa-Maria I, Varghese M, Ksiezak-Reding H, Dzhun A, Wang J, Pasinetti GM. Paired helical filaments from Alzheimer disease brain induce intracellular accumulation of tau protein in aggresomes. J Biol Chem 2012; 287: 20522–33. 156 Kfoury N, Holmes BB, Jiang H, Holtzman DM, Diamond MI. Transcellular propagation of tau aggregation by fibrillar species. J Biol Chem 2012; 287: 19440–51. 157 Holmes BB, Diamond MI. Cellular mechanisms of protein aggregate propagation. Curr Opin Neurol 2012; 25: 721–26. 158 Wu JW, Herman M, Liu L, et al. Small misfolded tau species are internalized via bulk endocytosis and anterogradely and retrogradely transported in neurons. J Biol Chem 2013: 288: 1856–70. 159 Liu L, Drouet V, Wu JW, et al. Trans-synaptic spread of tau pathology in vivo. PLoS One 2012; 7: e31302. 160 de Calignon A, Polydoro M, Suárez-Calvet M, et al. Propagation of tau pathology in a model of early Alzheimer’s disease. Neuron 2012; 73: 685–97. 161 Daebel V, Chinnathambi S, Biernat J, et al. β-Sheet core of tau paired helical filaments revealed by solid-state NMR. J Am Chem Soc 2012; 134: 13982–89. 162 Sievers SA, Karanicolas J, Chang HW, et al. Structure-based design of non-natural amino-acid inhibitors of amyloid fibril formation. Nature 2011; 475: 96–99. 163 Taniguchi S, Suzuki N, Masuda M, et al. Inhibition of heparin-induced tau filament formation by phenothiazines, polyphenols and porphyrins. J Biol Chem 2005; 280: 7614–23. 164 Crowe A, Huang W, Ballatore C, et al. Identification of aminothienopyridazine inhibitors of tau assembly by quantitative high-throughput screening. Biochemistry 2009; 48: 7732–45. 165 Bulic B, Pickhardt M, Mandelkow EM, et al. Tau protein and tau aggregation inhibitors. Neuropharmacology 2010; 59: 276–89. 166 Wang J, Santa-Maria I, Ho L, et al. Grape derived polyphenols attenuate tau neuropathology in a mouse model of Alzheimer’s disease. J Alzheimers Dis 2010; 22: 653–61. 167 Wischik CM, Edwards PC, Lai RY, et al. Selective inhibition of Alzheimer disease-like tau aggregation by phenothiazines. Proc Natl Acad Sci USA 1996; 93: 11213–18. 168 Akoury E, Pickhardt M, Gajda M, Biernat J, Mandelkow E, Zweckstetter M. Mechanistic basis of phenothiazine-driven inhibition of tau aggregation. Angew Chem Int Ed Engl 2013; 52: 3511–15. 169 O’Leary JC, Li Q, Marinec P, et al. Phenothiazine-mediated rescue of cognition in tau transgenic mice requires neuroprotection and reduced soluble tau burden. Mol Neurodegen 2010; 5: 45. 170 Congdon EE, Wu JW, Myeku N, et al. Methylthioninium chloride (methylene blue) induces autophagy and attenuates tauopathy in vitro and in vivo. Autophagy 2012; 8: 609–22. 171 Hosokawa M, Arai T, Masuda-Suzukake M, et al. Methylene blue reduced abnormal tau accumulation in P301L tau transgenic mice. PLoS One 2012; 7: e52389. 172 Wischik CM, Staff R. Challenges in the conduct of diseasemodifying trials in Alzheimer’s disease. J Nutr Health Aging 2009; 13: 367–69. 173 Le Corre S, Klafki HW, Plesnila N, et al. An inhibitor of tau hyperphosphorylation prevents severe motor impairments in tau transgenic mice. Proc Natl Acad Sci USA 2006; 103: 9673–78.

www.thelancet.com/neurology Vol 12 June 2013

174 Noble W, Planel E, Zehr C et al. Inhibition of glycogen synthase kinase-3 by lithium correlates with reduced tauopathy and degeneration in vivo. Proc Natl Acad Sci USA 2005; 102: 6990–95. 175 Hurtado DE, Molina-Porcel L, Carroll JC, et al. Selectively silencing GSK-3 isoforms reduces plaques and tangles in mouse models of Alzheimer’s disease. J Neurosci 2012; 32: 7392–402. 176 Hampel H, Ewers M, Burger K, et al. Lithium trial in Alzheimer’s disease: a randomized, single-blind, placebo-controlled, multicenter 10-week study. J Clin Psychiatry 2009; 70: 922–31. 177 Domínguez JM, Fuertes A, Orozco L, del Monte-Millán M, Delgado E, Medina M. Evidence for irreversible inhibition of glycogen synthase kinase-3β by tideglusib. J Biol Chem 2012; 287: 893–904. 178 Zeltia. Noscira announces results from ARGO phase IIb trial of tideglusib for the treatment of Alzheimer’s disease. Oct 11, 2012. http://www.zeltia.com/media/docs/esflsziq.pdf?ie=UTF8&oe=UTF-8&q=prettyphoto&iframe=true&width=100%&height=100% (accessed May 2, 2013). 179 Goedert M, Cohen ES, Jakes R, Cohen P. p42 MAP kinase phosphorylation sites in microtubule-associated protein tau are dephosphorylated by protein phosphatase 2A1. Implications for Alzheimer’s disease. FEBS Lett 1992; 312: 95–99. 180 Sontag E, Nunbhadki-Craig V, Lee G, Bloom GS, Mumby MC. Regulation of the phosphorylation state and microtubule-binding activity of tau by protein phosphatase 2A. Neuron 1996; 17: 1201–07. 181 Xu Y, Chen Y, Zhang P, Jeffrey PD, Shi Y. Structure of a protein phosphatase 2A holoenzyme: insights into B55-mediated tau dephosphorylation. Mol Cell 2008; 31: 873–85. 182 Wu J, Tolstykh T, Lee J, Boyd K, Dtock JB, Broach JR. Carboxyl methylation of the phosphoprotein phosphatase 2A catalytic subunit promotes its functional association with regulatory subunits in vivo. EMBO J 2000; 19: 5672–81. 183 Sontag JM, Nunbhadki-Craig V, Montgomery L, et al. Folate deficiency induces in vitro and mouse brain region-specific downregulation of leucine carboxyl methyltransferase-1 and protein phosphatase 2ABα subunit expression that correlate with enhanced tau phosphorylation. J Neurosci 2008; 28: 11477–87. 184 Kickstein E, Krauss S, Thornhill P, et al. Biguanide metformin acts on tau phosphorylation via mTOR/protein phosphatase 2A (PP2A) signalling. Proc Natl Acad Sci USA 2010; 107: 21830–35. 185 van Eersel J, Ke YD, Liu X, et al. Sodium selenate mitigates tau pathology, neurodegeneration, and functional deficits in Alzheimer’s disease models. Proc Natl Acad Sci USA 2010; 107: 13888–93. 186 Liu F, Iqbal K, Grundke-Iqbal I, Hart GW, Gong CX. O-GlcNAcylation regulates phosphorylation of tau: a mechanism involved in Alzheimer’s disease. Proc Natl Acad Sci USA 2004; 101: 10804–09. 187 Yuzwa SA, Shan X, Macauley MS, et al. Increasing O-GlcNAc slows neurodegeneration and stabilizes tau against aggregation. Nat Chem Biol 2012; 8: 393–99. 188 Evans CG, Jinwal UK, Makley LN, Dickey CA, Gestwicki JE. Identification of dihydropyridines that reduce cellular tau levels. Chem Commun 2011; 47: 529–31. 189 David DC, Layfield R, Serpell LC, Narain Y, Goedert M, Spillantini MG. Proteasomal degradation of tau protein. J Neurochem 2002; 83: 176–85. 190 Petrucelli L, Dickson D, Kehoe K, et al. CHIP and Hsp70 regulate tau ubiquitination, degradation and aggregation. Hum Mol Genet 2004; 13: 703–14. 191 Luo W, Dou F, Rodina A, et al. Roles of heat-shock protein 90 in maintaining and facilitating the neurodegenerative phenotype in tauopathies. Proc Natl Acad Sci USA 2007; 104: 9511–16. 192 Lee BH, Lee MJ, Park S, et al. Enhancement of proteasome activity by a small-molecule inhibitor of USP14. Nature 2010; 467: 179–84. 193 Sarkar S, Davies JE, Huang Z, Tunnacliffe A, Rubinsztein DC. Trehalose, a novel mTOR-independent autophagy enhancer accelerates the clearance of mutant huntingtin and α-synuclein. J Biol Chem 2007; 282: 5641–52. 194 Schaeffer V, Lavenir I, Ozcelik S, Tolnay M, Winkler DT, Goedert M. Stimulation of autophagy reduces neurodegeneration in a mouse model of human tauopathy. Brain 2012; 135: 2169–77. 195 Krüger U, Wang Y, Kumar S, Mandelkow EM. Autophagic degradation of tau in primary neurons and its enhancement by trehalose. Neurobiol Aging 2012; 33: 2291–305.

621

Review

196 Asuni AA, Boutajangout A, Quartermain D, Sigurdsson EM. Immunotherapy targeting pathological tau conformers in a tangle mouse model reduces brain pathology with associated functional improvements. J Neurosci 2007; 27: 9115–29. 197 Boime M, Grigoriadis N, Lourbopoulos A, Haber E, Abramsky O, Rosenmann H. Efficacy and safety of immunization with phosphorylated tau against neurofibrillary tangles in mice. Exp Neurol 2010; 224: 472–85. 198 Boutajangout A, Quartermain D, Sigurdsson EM. Immunotherapy targeting pathological tau prevents cognitive decline in a new tangle mouse model. J Neurosci 2010; 30: 16559–66. 199 Boutajangout A, Ingadottir J, Davies P, Sigurdsson EM. Passive immunization targeting pathological phospho-tau protein in a mouse model reduces functional decline and clears tau aggregates from the brain. J Neurochem 2011; 118: 658–67. 200 Chai X, Wu S, Murray TK, et al. Passive immunization with anti-tau antibodies in two transgenic models. Reduction of tau pathology and delay of disease progression. J Biol Chem 2011; 286: 34457–67. 201 Bi M, Ittner A, Ke YD, Götz J, Ittner LM. Tau-targeted immunization impedes progression of neurofibrillary histopathology in aged P301L tau transgenic mice. PLoS One 2011; 6: e26860. 202 Troquier L, Caillierez R, Burnout S, et al. Targeting phospho-Ser422 by active tau immunotherapy in the THY-Tau22 mouse model: a suitable therapeutic approach. Curr Alzheimers Res 2012; 9: 397–405. 203 Bhaskar K, Konerth M, Kokiko-Cochran ON, Cardona A, Ransohoff RM, Lamb BT. Regulation of tau pathology by the microglial fractalkine receptor. Neuron 2010; 68: 19–31. 204 Nash KR, Lee DC, Hunt JB, et al. Fractalkine overexpression suppresses tau pathology in a mouse model of tauopathy. Neurobiol Aging 2013; 34: 1540–48. 205 Bellucci A, Westwood AJ, Ingram E, Casamenti F, Goedert M, Spillantini MG. Induction of inflammatory mediators and microglial activation in mice transgenic for mutant human P301S tau protein. Am J Pathol 2004; 165: 1643–52. 206 Yoshiyama Y, Higuchi M, Zhang B, et al. Synapse loss and microglial activation precede tangles in a P301S tauopathy mouse model. Neuron 2007; 53: 337–51. 207 Chambraud B, Sardin E, Giustiniani J, et al. A role for FKBP52 in tau protein function. Proc Natl Acad Sci USA 2010; 107: 2658–63 208 Barten DM, Fanara P, Andorfer C, et al. Hyperdynamic microtubules, cognitive deficits, and pathology are improved in tau transgenic mice with low doses of the microtubule-stabilizing agent BMS-241027. J Neurosci 2012; 32: 7137–45.

622

209 Brunden KR, Zhang B, Carroll J, et al. Epothilone D improves microtubule density, axonal integrity and cognition in a transgenic mouse model of tauopathy. J Neurosci 2010; 30: 13861–66. 210 Zhang B, Carroll J, Trojanowski JQ, et al. The microtubulestabilizing agent, epothilone D, reduces axonal dysfunction, neurotoxicity, cognitive deficits, and Alzheimer-like pathology in an interventional study with aged tau transgenic mice. J Neurosci 2012; 32: 3601–11. 211 Shiryaev N, Jouroukhin Y, Giladi E, et al. NAP protects memory, increases soluble tau and reduces tau hyperphosphorylation in a tauopathy model. Neurobiol Dis 2009; 24: 381–88. 212 Allon Therapeutics Inc. Allon announces PSP clinical trial results. Dec 18, 2012. http://www.allontherapeutics.com/2012/12/allonannounces-psp-clinical-trial-results (accessed April 24, 2013). 213 de Calignon A, Fox LM, Pitstick R, et al. Caspase activation precedes and leads to tangles. Nature 2010; 464: 1201–04. 214 Delobel P, Lavenir I, Fraser G, et al. Analysis of tau phosphorylation and truncation in a mouse model of human tauopathy. Am J Pathol 2008; 172: 123–31. 215 Lin WL, Dickson DW, Sahara N. Immunoelectron microscopic and biochemical studies of caspase-cleaved tau in a mouse model of tauopathy. J Neuropathol Exp Neurol 2011; 70: 779–87. 216 Blurton-Jones M, Kitazawa M, Martinez-Coria H, et al. Neural stem cells improve cognition via BDNF in a transgenic model of Alzheimer disease. Proc Natl Acad Sci USA 2009; 106: 13594–99. 217 Hampton DW, Webber DJ, Bilican B, Goedert M, Spillantini MG, Chandran S. Cell-mediated neuroprotection in a mouse model of human tauopathy. J Neurosci 2010; 30: 9973–83. 218 Nahahara AH, Merrill DA, Coppola G, et al. Neuroprotective effects of brain-derived neurotrophic factor in rodent and primate models of Alzheimer’s disease. Nat Med 2009; 15: 331–37. 219 Wang JM, Singh C, Liu L, et al. Allopregnanolone reverses neurogenic and cognitive deficits in mouse model of Alzheimer’s disease. Proc Natl Acad Sci USA 2010; 107: 6498–6503. 220 Blanchard J, Wanka L, Tung YC, et al. Pharmacologic reversal of neurogenic and neuroplastic abnormalities and cognitive impairments without affecting Aβ and tau pathologies in 3xTg-AD mice. Acta Neuropathol 2010; 120: 605–21.

www.thelancet.com/neurology Vol 12 June 2013